Kinetic determination of L(-)malic acid in wines using sequential injection analysis

In the present work a sequential injection system for the enzymatic determination of L(-)malic acid in wines is described. The determination was based on the detection of NADH, formed by oxidation of L-malate to oxaloacetate, catalysed by L-malate dehydrogenase. A kinetic method was implemented by stopping the flow when a solution plug containing sample, NAD+ and enzyme reached the detector. The rate of reaction was measured by monitoring absorbance changes at 340nm during a fixed period of time. A linear calibration curve (rate of reaction versus concentration) was obtained for concentrations of L(-)malic acid between 0.01 and 0.15gl-1. The determination rate was about 22h-1 and samples were diluted 25 or 50 times before introduction to the system. Results obtained from 16 wine samples were comparable to those obtained by the reference method.